Ultrastructural features of monomorphic adenoma of the parotid gland.

A monomorphic adenoma of the parotid gland (basaloid type) was studied with the electron microscope. In general, the tumor consisted of a single cell type, except in areas of squamous metaplasia. The cells were polygonal with prominent nuclei and scant cytoplasmic organelles. Intercellular junctions and plasmalemmal projections (filopodia) were conspicuous. Myoepithelial cells and secretory granules were absent. On the basis of these ultrastructural features, it is proposed that the interlobular duct, rather than the intercalated duct, is the probable site of origin of this tumor.